In the manufacture of paper and board products in a twin fabric paper making machine, a highly aqueous stock consisting of about 98–99.8% water and from 0.2–2% papermaking fibers and other solids is ejected at high speed from a headbox slice into the gap between the moving forming fabrics. In the following discussion, for the sake of simplicity, it is assumed that the path followed by the twin forming fabrics is more or less horizontal; other arrangements are known and used in which the path is not horizontal and may even be vertical. In such machines a direction substantially perpendicular to the two forming fabrics is known as the Z-direction. Further, when the path is horizontal, one forming fabric is commonly referred to as the “upper”, and the other as the “lower” fabric.
In a twin fabric forming section, the two forming fabrics each pass in sliding contact over a plurality of fabric support elements which serve to define a path along which the two forming fabrics move together; these elements, together with their supporting structures, are generally known as forming shoes and the forming fabric path over them may be straight or curved. The stationary fabric support elements in contact with the machine side of the lower forming fabric may be fixed, or may be adjustable in the Z-direction. In current practice, the stationary support elements in contact with the machine side of the upper fabric are normally not adjustable, while those in contact with the machine side of the lower forming fabric can be resiliently mounted so as to assist with fluid removal. The resiliently mounted blades are not used in association with a vacuum box, and rely on gravity to assist with fluid drainage. Further, the fabric support elements in contact with each fabric are known to generate pressure pulses within the stock between the two forming fabrics as they move together under tension through the forming section. The nature of the pressure pulses generated in the stock can only be adjusted by changing fabric tension, by changing the angles of wrap of the fabrics over the fabric contacting surfaces of a chosen element or elements, by changing a chosen element or elements, or by physically removing an element or elements. Pressure pulsed benefit the forming process by causing relative motion in the stock layer that breaks apart the fibre flocs and causes the fibers to become more uniformly dispersed in the stock layer.
Fixed elements are unable to react to transient changes in the thickness of the stock layer between the forming fabrics. Such changes can occur when the machine is being started, and also may occur at any time during normal machine operation. In an extreme case, transient stock thickness variation can result in damage to either, or both, of the forming fabrics.
This invention seeks to provide a process, and an apparatus, by means of which the papermaker can control both fluid removal from the machine side surface of at least one of the forming fabrics, and pulse formation in the stock between the two forming fabrics so as to enhance, maintain or diminish the kinetic energy injected into the stock, thus allowing optimized fluid removal and agitation in accordance with paper making conditions. This invention further seeks to provide an apparatus in which a skimmer blade is resiliently mounted, so that the mounting allows the skimmer blade to respond to transient changes in the thickness of the stock layer between the two moving forming fabrics. The invention further seeks to provide an apparatus including a skimmer blade which is resiliently mounted and located immediately downstream of an elongate suction orifice attached to a vacuum box, wherein the resilient mounting is attached to the vacuum box, and the vacuum box as a unit can be moved away from the path of the forming fabrics as they move through the forming section.